Toxic bug has meat-eaters in its sights

时间：2017-08-13 02:00:06166网络整理admin

By Ewen Callaway Just when it seemed that contaminated vegetables posed a bigger risk of food poisoning than eating meat, along comes a pathogen that will only attack those of us who are carnivores. The bacterium – a strain of Escherichia coli – makes a toxin that does its worst by latching onto a sugar molecule that humans don’t have naturally. We can, only acquire it by eating red meat or dairy products. “This toxin originally evolved to attack cattle or some other animals,” says Ajit Varki, an expert in molecular medicine at the University of California, San Diego, who was involved in the study. By eating the toxin’s intended target we made ourselves vulnerable too, he says. When unlucky meat-eaters ingest this particular E. coli strain, its toxin kills the cells that line the gut, eventually causing bloody diarrhoea, Varki says. It also heads for blood vessels and the kidneys. Along with colleagues in Australia, Varki has discovered that the bacterium has a penchant for a particular version of a type of sugar called sialic acid. All animals adorn their cells with different sialic acids, which are important for cell-to-cell communication. Bacteria have evolved to recognise the sugars, and use them as gateways for injecting their toxins. Varki’s team discovered that one E. coli toxin attaches best to a sialic acid called GC. Molecular mapping of the toxin showed that it fits snugly with the sugar molecule to ease entry into the cell. Our ancestors lost the genes to produce GC some 2 million years ago, perhaps as a defence against an ape version of malaria. This leaves meat and dairy as the only source, Varki says. To demonstrate that humans could add the sugar to their own cells, Varki consumed some pure GC. “It showed up in my urine and got incorporated into my facial hair,” he says. “I stopped eating red meat after that.” He estimates that the average American ingests about 20 milligrams of GC each day. This could be enough to give the bacteria a pathway into the cells lining our gut, though Varki hasn”t tried that test on himself. He has, however, shown that human cells grown in the lab become far more sensitive to the toxin when GC is included in their growth medium. From this he reckons that people who eat red meat and dairy products will be about 10 times more susceptible to infection by this strain of E. coli than those who don’t. “It’s a sort of worst of all worlds if you’re a human and you eat some of this stuff,” says Paul Crocker, who investigates the biological role of sugars at the University of Dundee, UK. Animals that produce GC naturally have the sugar in blood serum, where it mops up the toxin and keeps it out of the gut. Varki suggests that other ailments could also be due to GC from meat and dairy as the immune system mounts a response against it. “We think other diseases associated with red meat – cancer, heart attack and autoimmunity – may be explained by this ongoing reaction.” Journal reference: Nature (DOI: 10.1038/nature07428) More on these topics: